1. Field of the Invention
The present invention generally relates to dielectrophoresis and its application in analytical devices and filtration technologies.
2. Description of the Related Art
Dielectrophoresis (DEP) is the motion of particles caused by the effects of conduction and dielectric polarization in non-uniform electric fields. Unlike electrophoresis, where the force acting on a particle is determined by its net charge, the dielectrophoretic force depends on the geometrical, conductive, and dielectric properties of the particle. A complex conductivity of a medium can be defined as σ*=σ+iω∈, where σ is the real conductivity and ∈ is the permittivity of the medium, i is the square root of −1, and σ* is the angular frequency of the applied electric field, E. According to well-known theory, the dielectrophoretic force is proportional to the differences in complex conductivity of the particle and suspending liquid and square of the applied electric field. Without being bound by theory, for a spherical particle of radius r, the DEP force, FDEP is given byFDEP=2πr3∈mRe[fCM]∇E2 where ∈m is the absolute permittivity of the suspending medium, E is the local (rms) electric field, ∇ the del vector operator and Re[fCM] is the real part of the Clausius-Mossotti factor, defined as:
      f    CM    =                    σ        p        *            -              σ        m        *                            σ        p        *            +              2        ⁢                  σ          m          *                    where σ*p and σ*m are the complex conductivities of the particle and medium respectively, as described in Hughes, et al. (1998) Biochimica et Biophysica Acta 1425: 119-126, which is herein incorporated by reference. Depending on the relative conductivities of the particle and medium, the Clausius-Mossotti factor can be positive, n resulting in a force toward stronger electric fields, or negative, resulting in a force away from stronger electric fields. The particle motion toward and away from stronger electric fields is called, respectively, positive and negative DEP.
Thus, when a particle is exposed to a non-uniform electric field, it experiences dielectrophoretic forces resulting from conduction and polarization that scale with the electric field intensity. The magnitude, sign, and phase of these forces depend on the frequency of the applied field and electrical properties of the particle and medium, such as conductivity, permittivity, morphology and shape of the particle. Thus dielectrophoresis can be used to sort and move particles selectively. See Pohl, H. A., J. Appl. Phys., 22:869-871; Pohl, H. A., Dielectrophoresis, Cambridge University Press (1978); Huang Y., R. C. Gascoyne et al., Biophysical Journal, 73:1118-1129; Wang X. B., Gascoyne, R. C., Anal. Chem. 71:911-918, 1999; and U.S. Pat. No. 5,858,192, all of which are hereby incorporated by reference.
Insulator-based (electrodeless) dielectrophoresis (iDEP) has been previously described and utilized for the selective concentration and separation of analytes in microfluidic devices. See Cummings and Singh (2003) Anal. Chem. 75:4724-4731, Lapizco-Encinas, et al. (2004) Electrophoresis 25:1695-1704, and Lapizco-Encinas, et al. (2004) Anal. Chem. 76:1571-1579, which are herein incorporated by reference. These devices use spatially nonuniform insulating structures to generate the nonuniform electric field needed to drive DEP. These iDEP devices are practically limited to processing microliter volumes and require microfabrication. Prior art devices and methods employing iDEP may be used effectively for systems that process such small volume samples, but are ineffective for real-time monitoring and analysis of large volumes and flows, e.g., flow rates greater than one liter of per hour.
Thus, a need exists for methods and devices that allow dielectrophoretic based assays of large volumes and high flow rates.